Abstract:

X–linked adrenoleukodystrophy (X–ALD) is the most common peroxisomal enzyme deficiency disorder, characterized by inborn mutations in the ABCD1 gene, an ATP–binding cassette (ABC) half–transporter. The ABCD1 gene encodes the adrenoleukodystrophy protein (ALDP), the transporter for the very–long–chain fatty acids (VLCFA; C > 22:0) from the cytosol into the peroxisomes to enter the peroxisomal B–oxidation pathway. The diagnostic disease marker is the elevated levels of VLCFAs which accumulate in different tissues and body fluids, leading to inflammatory demyelination, neuro–deterioration and adrenocortical insufficiency. At present, there is no satisfactory therapy for X–ALD available. However, another peroxisomal ABC half–transporter, ALDRP can compensate for the functional loss of ALDP and is encoded by the ABCD2 gene. This prompted a new approach to treatment strategies. Phenylbutyric acid (PBA) over–expresses the ABCD2 gene, leading to an increased expression of ALDRP and PBA decreases VLCFA levels by increasing peroxisomal B–oxidation. This study had a dual aim: to determine the antioxidant capacity of PBA and to verify known and identify new metabolites of PBA. In vitro, HeLa cells were cultivated and treated with 0.5 mM, 1 mM, 2 mM and 5 mM PBA for 48 hours. The ROS, lipid peroxidation, apoptosis and cell viability were determined using fluorescein–based flow cytometry. Images were taken to visualize the peroxisome proliferation. In vivo, a vervet monkey was given a single dose of 130 mg/kg PBA. Blood was collected before treatment and 15 minutes, 30 minutes, 1, 2 and 3 hours after treatment. ROS, apoptosis and lipid peroxidation were determined by fluorescein–based flow cytometry. Urine was collected before treatment and 15 minutes, 30 minutes, 1, 2, 3, 7 and 24 hours after PBA treatment. A standardised method, employing gas chromatography–mass spectrometry (GC/MS), was used to analyse the organic acids in the urine and fatty acids in the blood. In vitro results showed decreased levels of ROS and lipid peroxidation with increased concentrations of PBA. PBA showed a protective effect towards the HeLa cells with reduced apoptosis and a high number of viable cells. In vivo levels of ROS en lipid peroxidation decreased over time of treatment with PBA. The fluorescence microscope images confirmed an increased number of peroxisomes after PBA treatment. The short term effect of PBA showed an initial, but small decrease in the levels of the fatty acids, suggesting induction over a longer period rather than activation of peroxisomal B–oxidation. New metabolites of phenylbutyrate were identified in the urine of a vervet monkey. These new metabolites originated from monooxygenase, N–phenylacetyl–glutamine synthases and B–oxidation byproducts. Recently discovered metabolites in humans and rats were also verified and confirmed in the vervet monkey. We therefore propose that treatment with PBA, on account of its beneficial effects of restoring VLCFA levels and reducing oxidative stress, could be considered a novel approach for the treatment of X–ALD.